1. Field of the Invention
The present invention relates to an apparatus and method for evaluating a piezoelectric film, which evaluate either deformation or displacement of a piezoelectric film with a simple manner, efficiency, and high accuracy.
2. Description of the Related Art
To evaluate a slight deformation of a sample, such as evaluating the piezoelectricity thereof, laser displacement meters utilizing an interference of light, atomic force microscopes (AFM) and the like have conventionally been utilized. As shown in FIG. 1, butterfly curves are utilized to evaluate piezoelectricity of a piezoelectric film, and such the butterfly curves plot deformations of the piezoelectric film relative to the applied voltage thereto.
Specifically, in the case where the AFM is utilized for evaluating the piezoelectricity, a tip 7 of a probe 6 is placed onto a surface of a sample 5, which has a substrate 1, a lower electrode 2, a piezoelectric film 3, and an upper electrode 4 formed in this order on the substrate, as shown in FIG. 2A. Sequentially, the voltage is applied to the lower electrode 2 and the upper electrode 4. The deformation of the sample 5 at the time when the voltage is applied is measured as well as the displacement magnitude of the probe 6, and the results are plotted in a graph so as to obtain a butterfly curve as shown in FIG. 1.
At the time of detecting the deformation of the sample in such a devise using the AFM, there has been proposed a method wherein a laser beam is emitted from a light source to the probe whose tip is placed on the surface of the sample, and the reflected beam from the probe is detected by means of a detector to thereby determine the displacement magnitude of the probe and the piezoelectricity of the sample is evaluated based upon the determined displacement magnitude (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 06-258072).
In an evaluation device using the aforementioned laser displacement meter, a displacement magnitude can be measured, for example, by irradiating a laser beam from a light source 8 to a surface of the sample 5 that is formed in the aforementioned manner, and detecting the reflected light from the sample 5, as shown in FIG. 2B.
There has also been proposed a more simple device wherein a displacement magnitude of a probe is measured by using a piezoelectric resistor, instead of a laser beam (refer to JP-A 10-312592).
FIG. 3 is a schematic diagram of the aforementioned device for evaluating a piezoelectric film, which illustrates the deformation and displacement of the piezoelectric film 3 at the time of evaluating the piezoelectricity thereof, before and after an application of a voltage thereto. The sample 5 as shown in FIG. 3(1) is shrank (or stretched) in the thickness direction, and is stretched (or shrank) in the horizontal direction as a result of the voltage application, as shown in FIG. 3(2). In this way, the sample 5 is deformed. By plotting the thickness of the piezoelectric film as shown with an arrow “a” in FIG. 3, it is expected to form a curve which shows the actual deformation and displacement of the piezoelectric film, as shown with “a” in FIG. 4.
However, as shown in FIG. 3(3), the piezoelectric film 3 is, in fact, stretched (or shrank) in the inner direction by the voltage at the time of being shrank (or stretched) in the thickness direction. Therefore, a stress is caused between the piezoelectric film 3 and the substrate 1, and the whole area of the substrate and the sample are warped as shown in FIG. 3(3). For this reason, in the film sample 5 formed on the substrate 1, the deformation of the piezoelectric film 3 itself is added with the displacement due to the warp of the sample 5 (the displacement shown with the arrow b in FIG. 3(3)), and the plots of the measured deformation and displacement shows the curve as shown with c in FIG. 4. Therefore, the actual deformation magnitude of the sample 5 itself cannot be accurately evaluated.
In order to solve these problems caused by a warp of a sample, there has been applied a method wherein a surface perpendicular axis on both surfaces of a sample is measured by a laser displacement meter, and a displacement magnitude of the sample is measured based upon the difference in the measurements. This method is called a double-beam system. In the double-beam system, the reflected light from a surface of the sample is lead to emit the other surface of the sample, and make reflected by the other surface. The light pass differences between the surface and the other surface of the sample caused as a result of the displacement of the sample are canceled, and thus only a thickness of the sample can be measured (refer to Rev. Sci. Instrum, 67. 1935 (1996), and FIG. 5).
However, there has been a problem in this method such that the control of the measuring positions is difficult. Specifically, in the laser displacement meter, it is necessary to irradiate a laser beam perpendicularly to a surface to be measured. At the time of aligning the laser beam, the incident angle to the sample changes if the area to be irradiated changes. It is therefore necessary to control the laser beam so as to irradiate to the certain area with the certain incident angle, but this control is extremely complicated. Especially, the control of the laser beam having a complicated and long optical path, e.g., a laser beam in the double-beam system, requires sophisticated technology. This may be overcome in accordance with a method which is capable of controlling the irradiating area and incident angle at once by preparing a special optic system. However, in this case, it is more time-consuming and troublesome to repair the system once the system becomes out of the order due to any change in the age.
In the simple laser displacement meter, either the single-beam system or the double-beam system, there were problems such that the maximum surface resolution starts decreasing at micrometer-order due to diffraction limit of the laser beam, compared to AFM, as shown in the following table 1; it is necessary to increase a numerical aperture of the condensing lens to improve the surface resolution, namely to shorten a focal length; and it is difficult to handle since the focal length is short.
Moreover, in the double-beam system, there are problems such that, together with the aforementioned problems of the surface resolution of micrometer-order, it is difficult to scan a laser beam to a sample; and it is not able to evaluate a three-dimensional structural deformation by measuring the displacement magnitude of the sample having a small structure of micrometer-order at various position by scanning a laser beam such as by means of a micro machine of Micro Electro Mechanical System, MEMS.
TABLE 1Laserdisplacement meterAFMVertical resolution40 pm/1 pm10 pm/1 pm or less(normal/max)Max. surfacemicrometerapproximatelyresolution20 pm or lessSurface scanDifficultEasy(3-dimensionalmeasurement)ControlComplicatedEasy(position, angle)
Accordingly, it has been desired, but not realized, to provide an apparatus and method for evaluating a piezoelectric film, which evaluate deformation and/or displacement of an dielectric film with a simple manner, efficiency, and high accuracy, without complex controls such as controls of positions and angles.
The present invention is therefore aimed at solving the aforementioned problems in the related art, and achieving the following object.
An object of the present invention is to provide an apparatus and method for evaluating a piezoelectric film, which evaluate a deformation and/or displacement of an dielectric film with a simple manner, efficiency, and high accuracy.
As a result of the diligent studies conducted by the present inventors to achieve the aforementioned object, the present inventors has found the following insights. Namely, it is an insight that, by measuring displacements of a piezoelectric film from both surfaces thereof by using two probes, the displacement magnitude of the piezoelectric film itself can be highly accurately evaluated based upon the deference in the measured displacements.
It is another insight that, by using the probes, the handling becomes easy, and an evaluation of the piezoelectric film can be easily and efficiently performed since a control of an incident angle which is required in a laser displacement meter is not necessary.
It is yet another insight that AFM of even a simple mechanism easily attains a high vertical resolution and surface resolution of nanometer-order compared with a laser displacement meter, as shown in Table 1, it is thus possible to locally measure the displacement, and it is also possible to measure the displacement of the sample three-dimensionally by scanning the measuring points within the surfaces of the sample.